The missing Malaysian Airlines flight which disappeared over the South China Sea last weekend with 239 people on board was seized and re-routed to the Indian Ocean, possibly up to Kazakhstan in Central Asia, Malaysia's Prime Minister Najib Razak said on Saturday.

He had ordered his search operation to switch from the South China Sea, where the plane's position was last clearly recorded, to the Indian Ocean, after new information confirmed the plane had been turned back en route from Kuala Lumpur to Beijing by people on-board who had deliberately switched off its radar and communications systems.

His comments, which were made in a statement to journalists at an airport hotel, stopped short of calling the seizure of the plane a “hijack” but stressed the investigation would now focus on the passengers and crew members to identify suspects.

Foreign intelligence agencies are now expected to assist in sifting through the passenger lists to identify suspects.

There is some suspicion of the pilot and co-pilot because they are regarded as the most likely to have the specialist aviation expertise to locate and switch off radar, satellite and other transponders to remove the aircraft from the 'grid' before changing its direction.

But investigators said there was no evidence implicating members of the crew and it was possible that some of its passengers also had the knowledge required.

Mr Razak said his government had put the search for the plane above national security, and that it was sharing sensitive radar data with neighbouring countries because it faced a situation "without precedent".

More than 40 ships and 50 aircraft from 14 countries had joined the search which had initially focused on the South China Sea where its last clear position was confirmed close to Vietnamese air space. The search was extended to the Strait of Malacca and the Andaman Sea after new, credible information from the Royal Malaysian Air Force indicated the plane had "turned back."

Early on Saturday morning, he said, he received new information from his search and investigation agencies, which confirmed that it had turned back and its communication systems were manually switched off to conceal its change of direction.

"Based on new satellite information, we can say with a high degree of certainty that the Aircraft Communications Addressing and Reporting System was disabled just before the aircraft reached the East coast of the Malaysian peninsular. Shortly afterwards, near the border between Malaysian and Vietnamese air traffic control, the aircraft’s transponder was switched off.

"From this point onwards, the Royal Malaysian Air Force primary radar showed that an aircraft which was believed – but not confirmed – to be MH370 did indeed turn back. It then flew in a westerly direction back over the Malaysian peninsular before turning northwest. Up until the point at which it left military primary radar coverage, these movements are consistent with deliberate action by someone on the plane," Mr Najib said.

Investigators are now assessing the date to calculate how far the plane could have travelled from its last recorded position at 08.11am on March 8, and in which direction.

"In view of this latest development the Malaysian authorities have refocused their investigation into the crew and passengers on board. Despite media reports that the plane was hijacked, I wish to be very clear: we are still investigating all possibilities as to what caused MH370 to deviate from its original flight path," he said.

Azharuddin Abdul Rahman, director-general of Malaysia's civil aviation authority, earlier told The Telegraph that his investigation had not found any evidence yet to establish the seizure of the plane was a classic hijacking or that any members of the crew had been involved in it.

"We're looking at every profile of the passengers and the crew but there is no firm evidence or leads so far," he said.

Western sources in Malaysia said the seizure could be a hijack but the government was reluctant to use the term until it had established the motives of those who had carried out the operation.

"No one knows who was responsible or what the motivation was ... The sensitivity is any political reasons or terrorism," he said.

Islamic terrorism carried out by Malaysian jihadists is regarded as unlikely as Malaysia has a tiny number of Muslim fundamentalists.

Malaysia has worked closely with Britain, the United States and other countries which have faced Islamic terrorism and is expected to collaborate closely in profiling all of the passengers and crew members of flight MH370 to identify suspects.

"The search for MH370 has entered a new phase. Over the last seven days, we have followed every lead and looked into every possibility. For the families and friends of those involved, we hope this new information brings us one step closer to finding the plane," said Mr Najib.